Alloy steel for use at high temperatures



Wesley R. Kegerise, Reading,

penter Steel Company, New Jersey Pa., assignor to The Car- Reading, Pa., a corporation of No Drawing. Application June 25, 1956, Serial No. 593,330

6 Claims. (Cl. 75-125) This invention relates to a hardenable alloy steel having good corrosion resistance, particularly at elevated temperatures, good toughness at ordinary temperatures, and a long stress rupture life with good ductility at elevated temperatures, and to articles made therefrom.

The alloy steel of my invention is suited for various other uses, but is particularly adapted to meet the ever increasing requirements demanded of steam turbine parts such as blades, buckets, bolts, etc. Such parts have been subjected to higher and higher steam operating temperatures and frequently are operated today at temperatures in the range of 800 to 1200 F. To be suitable for such use, the turbine parts must, of course, have good corrosion resistance at these temperatures and because they are subjected to high pressures at these high temperatures, must also have as desirable a stress rupture life as possible. In addition, such an alloy should not become embrittled but should be ductile and retain its ductility under these operating conditions for long periods of time; it should be capable of being annealed so that it can be easily machined at room temperature; it should be made of reasonably low cost ingredients; and it should also have good toughness or impact properties at room temperature.

In order to achieve as many of these properties as possible, alloy steels have heretofore been used containing around 12% chromium, along with smaller amounts (less than 10%) of one or more other elements such as nickel, tungsten, molybdenum, vanadium and cobalt. Each of these known alloys, however, fails to measure up completely to present day requirements, and has one or more disadvantages.

I have found that an alloy can be prepared which meets all of these demands to a high degree by incorporating in a 12% chromium steel small proportions of molybdenum, copper and nitrogen. By proper adjustment and control of this composition, a product can be made which after hardening is essentially martensitic (contains less than 15% of free ferrite), which has good toughness at room temperature as indicated by the V notch Charpy impact test, which has excellent stress rupture characteristics at elevated temperatures, e. g. at 1200 F., which is of relatively low cost to manufacture and which can be annealed soft enough for easy machining. For example, such an alloy can be prepared so that it is readily machineable when annealed but can be easily hardened and drawn to a Rockwell C hardness of about 27 to 33, and, in that condition, has an impact value of 30 to 50 footpounds when subjected to the standard V notch Charpy impact test. The same alloy, when given a stress rupture test under a 20,000 p. s. i. load at 1200 R, will have a life of at least 50 hours and frequently more than 65 hours with an elongation in 1 inch of at least 30% and a reduction in area of 60% or more.

2,816,830 Patented Dec. 17, 1 957 ice The composition which I have found to be capable of producing these results is as follows:

Balance substantially all iron, except for the usual amounts of phosphorous, sulfur and other impurities in such steels.

Small or normal residual amounts of nickel in the order of 0.1 or 0.2% are not objectionable but more than 0.5% nickel causes difiiculty in annealing the alloy. Substantial amounts, such as more than 0.1% of strong carbide forming elements, such as tungsten and vandium, should be avoided, however, because they are apt to form carbides at the interfaces of the martensitic matrix and free ferrite grains and possibly produce brittleness after long time exposure to high temperatures and pressures.

The carbon content of the alloy can be varied in accordance with the proportions of the other elements. However, in general, at least 0.1% carbon should be used so that the proportion of free ferrite in the alloy after hard.- ening will not be too high. The upper limit for carbon is not critical but should not be so high that the impact value at room temperature is seriously reduced, or the elongation at elevated temperatures is reduced to too low a point. With more than 0.30% carbon, there is also a greater likelihood of forming undesirable carbides which would adversely affect the properties of the alloy as by leading to the development of cracks or brittleness after long periods of usage at elevated. temperatures. The manganese and silicon ranges are selected for good melting practice, and chromiumin the range of 914% is needed for good corrosion resistance. It more than 14% chromium is used, there is a strong tendency for production of free ferrite in undesirably large amounts. Usually the optimum results are obtained with about 10.5-12.5 chromium.

The basic properties of the alloy are substantially improved by the use of 24% molybdenum, and the stress rupture properties are greatly improved by the inclusion of 0.06% or more of nitrogen. So far as is known, the maximum amount of nitrogen that can be employed is limited only by the amount that can be incorporated in the melt and held in solution, still producing sound steel. Amounts of nitrogen up to 0.16% can be incorporated by the ordinary methods of adding nitrogen in preparing alloys in the arc furnace. Somewhat larger amounts of nitrogen, however, may be incorporated by using special techniques. It has been known for sometimes that nitrogen acts as a strengthener at elevated temperatures in austenitic stainless steel alloys but little is known about the effect of nitrogen on the martensitic steel alloys. Contrary to expectations, I found that the increased nitrogen content, particularly when combined with the lower carbon content in the range given, not only gave very desirable elongation and stress rupture life at elevated temperatures but did not reduce the toughness at room temperatures. In fact, at room temperature the impactproperties of the alloycontaining this combination of elements was actually substantially improved.

The addition of 13% copper greatly improves-thestress rupture life of the alloy at elevated temperatures .3 such as 1200 F. Without the copper, this alloy has the other desired properties but at the expense of a reduced stress rupture life. By including the copper, the stress rupture life of the alloy is raised to a high level and this is accomplished without adversely affecting the other desirable properties mentioned above.

In order that my invention may be better understood, the following examples of alloy steels prepared according to this invention are given for the purposes of illustration.

Example I.-An alloy was prepared having the following analysis:

Percent Carbon 0.23 Manganese 0.66 Silicon 0.3 6 Chromium 1 1.5 Molybdenum 2.5 Nitrogen 0.083 Copper 1.8 Iron Balance In addition, this alloy also contained 0.09% nickel, .008% phosphorous and .011% sulfur.

The alloy, for test purposes, was cast into a small 4" ingot and hammered to a 1" square bar. The heat treating cycle involved normalizing the sample for 2 hours at 1700 F., soaking for 1 hour at 1900 F. and then oil-quenching to harden the product. The quenched product was then drawn by reheating to 1300 F. for 2 hours. In this condition, the sample showed a completely drawn martensitic micro-structure, had a Rockwell C hardness of 33 and gave values of 34-42 footpounds when subjected to the V notch Charpy impact test. Samples were also subjected to stress rupture tests under a load of 20,000 p. s. i. at 1200 F. This particular alloy, when subjected to this test, had a life of 67.6 hours and showed an elongation of 47.2% with a reduction in area of 81%.

Example II.-An alloy was prepared having the following analysis:

Percent Carbon 0.21 Manganese 0.73 Silicon 0.25 Chromium 11.46 Molybdenum 2.67 Nitrogen 0.079 Copper 2.1 Iron Balance This alloy in addition contained 0.06% nickel, 0.006% phosphorous and .015 sulfur.

When subjected to the same heat treating cycles as specified in Example I, the product was completely martensitic in structure after drawing, had a Rockwell C hardness of 28 and gave impact values of 41-43 foot-pounds in the V notch Charpy impact test. The stress rupture tests under 20,000 p. s. i. load at 1200 F. gave a life of 123 hours and showed a total elongation of 40.6% with a reduction in area of 66.6%

Example lII.An alloy was prepared having the following analysis:

Percent Carbon 0.21 Manganese 0.77 Silicon 0.34 Chromium 11.4 Molybdenum 2.7 Nitrogen 0.079 Copper 1.2 Iron Balance This alloy contained, in addition, about 0.06% nickel, 0.006% phosphorous and 0.015% sulfur. After being subjected to the same heat treatment as in Example I, the product was found to have a drawn martensitic microstructure, except for about 3% of free ferrite, had a Rockwell C hardness of 27 and gave impact values of 42-46 foot-pounds in the V notch Charpy test. The stress rupture tests under 20,000 p. s. i. at 1200 F. showed a life of 86.2 hours with an elongation in 1 inch of 44% and a reduction in area of 80.4%.

Example IV.An alloy was prepared having the following analysis: 7

Percent Carbon 0.11 Manganese 0.72 Silicon 0.22 Chromium 11.4 Molybdenum 2.7 Nitrogen 0.079 Copper 2.1 Iron Balance In addition, this alloy contained 0.1% nickel, 0.005% phosphorous and 0.017% sulfur.

This alloy was subjected to the same heat treating cycle as in Example I except that it was drawn at 1250 F. for two hours instead of at 1300" F. because of the difference in carbon content. It then had a Rockwell C hardness of 27 with Charpy impact values of 41-50 foot-pounds. This alloy as drawn was martensitic in structure except for about 10% of free ferrite.

When subjected to stress rupture tests at 1200 F. under load of 20,000 p. s. i., the samples had a life of 58.6 hours and showed 33.2% elongation with a reduction in area of 71%.

The terms and expressions which I have employed are used as terms of description and not of limitation, and I have no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but recognize that various modifications are possible within the scope of the invention claimed.

I claim:

1. A steel alloy capable of being hardened to show an essentially martensitic structure, said alloy having good corrosion resistance, ductility and life at elevated temperatures and having high impact strength and being readily machineable at room temperature, and said alloy containing about 0.10-0.30% carbon, silicon in an amount not more than 0.5, 0.5- 1.5% manganese, 9-14% chromium, 2-4% molybdenum, 13% copper, nitrogen in an amount from 0.06% up to the amount that can be retained in solution and still produce sound steel, and the balance iron except for impurities usually associated with these elements.

2. A steel alloy having a structure as drawn that is martensitic and contains less than about 15% of free ferrite, said alloy having a V notch Charpy impact value of at least 30 foot-pounds at a Rockwell hardness of about 30 on the C scale, and having a stress rupture life under a load of 20,000 p. s. i. at 1200 F. of at least 50 hours with an elongation of at least 30%, said alloy containing about O.100.30% carbon, silicon in an amount not more than 0.5%, 0.5-1.5% manganese, 9-14% chromium, 2-4% molybdenum, 13% copper, nitrogen in an amount from 0.06% up to the amount that can be retained in solution and still produce sound steel, and the balance principally iron.

3. A steam turbine part exposed to high temperature and pressure steam such as a blade, bucket or the like, said part consisting of an essentially martensitic alloy having good corrosion resistance, ductility and life at elevated temperatures and having high impact strength and being readily machineable at room temperature, said alloy containing about 0.100.30% carbon, silicon in an amount not more than 0.5%, 0.51.5% manganese, 9-14% chromium, 2-4% molybdenum, 1-3% copper, nitrogen in an amount from 0.06% up to the amount that can be retained in solution and still produce sound steel, and the balance iron except for impurities usually associated with these elements.

4. A steel alloy containing carbon 0.10-0.30%, silicon in an amount not more than 0.5%, manganese 0.5-1.5%, chromium 9-l4%, molybdenum 2-4%, copper l-3%, nitrogen 0.060.16%, and the balance substantially all 1mm.

5. A steel alloy containing carbon 0.15-0.25%, silicon 0.20-0.50%, manganese 05-15%, chromium 10.5- 12.5%, molybdenum 23%, copper 1-2.5%, nitrogen 0.060.16%, and the balance substantially all iron.

to give an essentially martensitic structure, said alloy consisting essentially of carbon about 0.23%, silicon about 0.36%, manganese about 0.66%, chromium about 11.5%, molybdenum about 2.5%, copper about 1.8%, nitrogen about 0.08%, and the balance substantially all iron.

References Cited in the file of this patent FOREIGN PATENTS 6. A steel alloy capable of being hardened and drawn 10 475895 Great Britain 1937 

1. A STEEL ALLOY CAPABLE OF BEING HARDENED TO SHOW AN ESSENTIALLY MARTENSITIC STRUCTURE, SAID ALLOY HAVING GOOD CORROSION RESISTANCE, DUCTILITY AND LIFE AT ELEVATED TEMPERATURE AND HAVING HIGH IMPACT STRENGHT AND BEING READILY MACHINEABLE AT ROOM TEMPERATURE, AND SAID ALLOY CONTAINING ABOUT 0.10-0.30% CARBON, SILICON IN AN AMOUNT NOT MORE THAN 0.5, 0.5-1.5% MANGANESE, 9-14% CHROMIUM, 2-4% MOLYBDENUM, 1-3% COPPER, NITROGEN IN AN AMOUNT FROM 0.06% UP TO THE AMOUNT THAT CAN BE RETAINED IN SOLUTION AND STILL PRODUCE SOUND STEEL, AND THE BALANCE IRON EXCEPT FOR IMPURITIES USUALLY ASSOCIATED WITH THESE ELEMENTS. 